The present invention relates to means for fastening a centre conductor in a cavity filter.
When constructing and manufacturing high-frequency filters for radio base stations, the filters are built-up to form so-called cavity filters that consist of a plurality of cavities, either with a separate centre conductor in each cavity or with more than one centre conductor per cavity. These filters are used, for instance, in base stations for GSM-based mobile telephony at the frequencies of 900 MHz and 1800/1900 MHz.
Each cavity and its center conductor/conductors functions as an electric oscillating circuit that can be represented by a parallel oscillation circuit having an inductive part L and a capacitive part C when the filter is tuned to a quarter wavelength of the received signal. The inductive part is determined essentially by the length of the centre conductor, while the capacitive part is determined essentially by the diameter of the centre conductor and its distance from the cavity side walls and a trimming plate provided on the cavity. When dimensioning a cavity filter, the filter frequency determines the length of the centre conductor principally at xcex/4. However, fxcx9c1/LC applies with respect to the inductance and capacitance of the oscillating circuit. Thus, the inductive part, and consequently the length of the centre conductor, can be reduced at a given frequency, by correspondingly increasing the capacitive part. The oscillations in a cavity generate an electromagnetic field that induces current in an adjacent cavity so that oscillation will also occur therein, therewith enabling the precise oscillation frequency to be adjusted with the aid of the trimming plate. It is evident from this that high requirements must be placed on the centre conductor with respect to its construction and with respect to the manner in which it is mounted in the cavity bottom.
The electric currents induced in the cavity flow along the length of the centre conductor and cross the cavity bottom and up along the sides of the cavity. In the case of a construction of this nature, the current is greatest at the junction between the centre conductor and the cavity bottom. At the high frequencies concerned, surface effects occur that cause the current to be conducted essentially closest to the surface. The high current and the reduction in the cross-sectional area for current conduction caused by said surface effects also results in an increase in temperature at the contact surface. This results, in turn, in mechanical stresses caused by the various states of the material at elevated temperatures. It is therefore necessary for the mutually contacting surfaces of the centre conductor and the cavity bottom to exhibit good contact properties. This is achieved by working the contact surfaces in a manner which will ensure that a high degree of flatness or planarity is obtained, preferably with the aid of a material that has good electrical conductivity, and by producing the centre conductor from a material whose coefficient of linear expansion is the same as that of the cavity-defining body, so as to provide a positive and reliable electrical contact junction even at elevated temperatures.
It will be evident from the aforegoing that one important aspect of the function of the cavity filter is that the filter will fit effectively between the centre conductor and the cavity bottom. This implies both a stable mechanical attachment and a good electric contact. It must be possible to mount and center the centre conductor in the cavity bottom very precisely and, at the same time, in the simplest possible manner.
An example of an earlier known method of connecting a centre conductor to a cavity filter is shown in FIG. 1. FIG. 1 is a cross-sectional illustration of a centre conductor 13 disposed in a cavity defined by a body 11 and having a bottom 12. In this known solution, the centre conductor 13 is hollow so that it can be screwed to the cavity bottom 12 by means of a screw 14 that is inserted through respective openings in the bottom surface of the centre conductor and the cavity bottom. The electrical junction between the center conductor and the cavity bottom consists of the bottom surface of the centre conductor that lies around the screw.
The present invention addresses the problem of providing better means for fastening a centre conductor in a cavity filter.
A first object of the present invention is to provide between the centre conductor and the cavity bottom a junction that includes a surface which has good physical and electrical contact properties so as to be able to obtain at the junction location a low impedance which also includes a low resistance.
Another object of the present invention is to provide a centre conductor that can be easily mounted and that fulfils the high precision requirements concerning its seating on the cavity bottom and also the aforesaid electrical contact properties at the junction location between the centre conductor and the cavity bottom.
These objects are achieved in accordance with the invention with the aid of fastener means that is integral with the centre conductor, such that the fastener means and said centre conductor form a single-piece structure. The contact surface between the centre conductor and the cavity bottom will preferably be surface-treated with a material of low resistivity.
A first advantage afforded by the inventive arrangement resides in stable mechanical attachment of the centre conductor and good electrical contact between the centre conductor and the cavity bottom, and also that accurate mounting of the centre conductor is facilitated.
A second advantage afforded by the inventive arrangement is that the centre conductor and its integrated fastener means can be manufactured in the same manner as a traditional screw for instance, which is a well known manufacturing technique. The manufacturing costs are thus relatively low.